The invention relates to a gas generator for a safety system, in particular for a vehicle occupant restraint system.
A gas generator known from the DE 197 26 276 comprises a combustion chamber which contains propellant, a liquid chamber which contains liquid, a mixing chamber in which a gas developed on burning of the propellant and the liquid mix with each other, and a liquid guide surface onto which the liquid when released is applied substantially tangentially. In conventional gas generators based on solid material, the solid propellant consists hitherto predominantly of sodium azide which, however, is hazardous from a toxic and ecological point of view, for which reason gas generators based on sodium azide are being increasingly replaced by gas generators which are free of sodium azide. The propellant here can consist of various compounds, a higher combustion temperature generally being achieved with these propellants compared with sodium azide, to avoid undesired combustion products. For this reason, the extremely hot gas must, however, be cooled more intensively until entry into the safety system, preferably a gas bag, so that the safety system, in particular the gas bag fabric, is not damaged. For this reason, so-called hybrid gas generators have been considered, which operate partly with a liquid which can also be combustible. The liquid is injected into the hot gas, so that this burns or evaporates and hence increases the volume. With non-burning liquids, the gas temperature is thereby greatly reduced. So that as quickly as possible as much liquid as possible evaporates or burns, this must have as large a surface as possible. The DE 197 26 276 proposes for this to apply the liquid in tangential direction onto a tube which widens in a trumpet shape. Thereby, as thin, uniform a liquid film as possible should spread out on the surface. The gas stream from the combustion chamber strikes onto the surface at an acute angle. This gas stream presses the liquid stream which runs close to and parallel to the surface of the tube widening in a trumpet shape, onto the surface of the tube. On the surface, the liquid is evaporated by the hot gas stream and arrives via outflow openings in the housing at the safety device.
The invention provides a gas generator in which the liquid evaporates or is burned even more quickly, so that the gas generator has a greater efficiency. This is achieved in a gas generator which comprises a combustion chamber which contains propellant, a liquid chamber which contains liquid, and a mixing chamber in which a gas developed on burning of the propellant and the liquid mix with each other. The gas generator further comprises a liquid guide surface onto which the liquid when released is applied substantially tangentially, and a gas directing channel and a surface delimiting the gas directing channel. A flow break-off edge is provided in the mixing chamber, which flow break-off edge is defined on one side by the liquid guide surface and on the other side by the surface delimiting the gas directing channel. The gas directing channel directs a stream of developed gas into the mixing chamber at an angle of between approximately 60xc2x0 and 120xc2x0 with respect to the liquid guide surface in a region of the flow break-off edge, so that the liquid loosing from the flow break-off edge is entrained by the gas stream. In the gas generator according to the invention, the streams or flows coming from the various sides of the flow break-off edge meet at the flow break-off edge, namely on the one hand the liquid and on the other hand the hot gas. The hot gas entrains the liquid in the direction of the outflow openings, swirls it and divides it into finest droplets. The liquid is therefore not directed onto a surface onto which the gas jet strikes and on which it evaporates, but rather the liquid is already intensively distributed beforehand by the gas stream, and can also evaporate or burn before striking onto another surface. In this respect, the invention also differs from a gas generator which operates with a so-called baffle plate, onto which a stream of liquid is directed. The liquid is divided into fine droplets by striking onto the baffle plate and not, as in the invention, by the two streams of gas and liquid which are directed differently.
Preferably, the gas stream and the liquid meet each other at an angle of 90xc2x0, by the gas stream forming an angle of approximately 90xc2x0 with respect to the liquid guide surface in the region of the flow break-off edge. As the liquid guide surface, as will be further explained later, can also run in a curved shape, the region is crucial in which the liquid leaves the liquid guide surface, i.e. in the region of or adjacent to the flow break-off edge.
When the gas flow is applied to the gas directing channel, which is preferably the case, and the channel therefore determines the direction of the gas stream in the region of the flow break-off edge, the direction of the gas stream can also be defined directly by the alignment of the gas directing channel. In this case, the inner face of the gas directing channel preferably ends at an angle of approximately 90xc2x0 to the liquid guide surface in the region of the flow break-off edge.
When all the adjoining surfaces forming the flow break-off edge are curved, the region of the surface close to the flow break-off edge is determinative for the angle which they form with respect to each other. If necessary, tangential planes must be formed in order to determine the angle.
According to the preferred embodiment, the liquid guide surface and the gas directing channel are aligned to each other in such a manner that the stream comprised of gas and entrained liquid is directed into the interior of the mixing chamber. This means that the stream of gas and liquid is not for instance directed directly onto a baffle plate or onto an adjacent wall, as was the gas in the generic prior art. The stream of gas and liquid is to have as long a path as possible through the mixing chamber, before it strikes onto another part. Thereby, also the time available is increased up to striking onto a wall, in which the gas can heat the liquid and can evaporate.
A development also serves the latter purpose, according to which the gas arriving from the combustion chamber into the mixing chamber flows through the mixing chamber in one direction. The liquid guide surface faces away from this direction of flow in the region of the flow break-off edge. This prevents the gas flow from pressing the liquid against the liquid guide surface. Rather, the liquid is to be carried away from the liquid guide surface by the gas stream, for which reason this guide surface also faces away from the direction of flow.
The gas flows through the mixing chamber substantially in axial direction. The liquid guide surface runs in radial direction in the region of the flow break-off edge, so that the two streams of gas and liquid meet each other at the preferred 90xc2x0 angle. So that the layer of liquid situated on the liquid guide surface, which layer flows to the flow break-off edge, is as thin as possible, the liquid guide surface widens towards the flow break-off edge in a conical shape. Thereby, the thickness of the liquid layer is reduced and makes possible a finer atomization of the liquid. A development serves for this, according to which the liquid guide surface curves outwards in a trumpet shape towards the flow break-off edge. This curved surface can deflect the stream of liquid for example through 90xc2x0, and namely from an entry region of the liquid in which, tangentially to the liquid guide surface, the liquid is directed onto the latter, up to the flow break-off edge.
The liquid is injected into the mixing chamber, a working space being provided which adjoins the liquid chamber. Gas is introduced into this working space, to express the liquid.
According to the preferred embodiment, a magnetic valve device is provided, which controls the quantity of gas arriving into the working space. Through the magnetic valve, which has a switching time of only 3 milliseconds, the quantity of liquid, hence the cooling of the gas stream and the entire quantity of gas, can be controlled. Also the chronological progress of the emerging quantity of gas and hence the gas pressure can be varied. Also, a so-called pulsing, i.e. an opening and closing of the valve, is possible. Furthermore, the variability of the gas generator is increased by simple means. For different purposes or different gas bags with various vehicles, also different quantities of gas or different gas pressure profiles are necessary to achieve an optimum restraining effect. Merely through a different controlling by means of different programming of the control unit, which is responsible for actuating the magnetic valve, differing quantities of gas and pressure profiles can be achieved with the same gas generator.
The expressing of the liquid can take place by means of a displaceable piston which separates the liquid chamber from the working space.
Usually, a portion of the generated gas is used for expressing the liquid from the liquid chamber. For this, for example, a pressure equalizing tube can be provided in a tubular gas generator. This axial pressure equalizing tube connects the combustion chamber with the working space with regard to flow. A preferred arrangement of the chambers and spaces with each other makes provision that the combustion chamber and the liquid chamber are arranged at opposite axial ends of the tubular gas generator and the mixing chamber is arranged therebetween. A liquid duct directs liquid in the direction of the combustion chamber. The pressure equalizing tube here can also at least partially delimit the liquid duct. For this, provision is made that the pressure equalizing tube is surrounded by a radially outer delimiting tube and therebetween a liquid duct is formed in the form of an annular channel. A guide system for liquid and gas is thus formed in a simple manner.
The liquid guide surface preferably adjoins the exterior of the pressure equalizing tube in the region of the ends of the pressure equalizing tube and of the delimiting tube on the combustion chamber side. Thereby, it is to be achieved that the flow is applied as tangentially as possible to the liquid guide surface or runs parallel thereto and is directed by it.
A structurally very simple development of the liquid guide surface is achieved by the surface being formed by the side of a delimiting wall, facing the mixing chamber, between combustion chamber and mixing chamber. This delimiting wall can run in a curved shape, viewed in cross-section, and can also narrow on the combustion chamber side up to the opening of the centrally arranged pressure equalizing tube. Thereby, the delimiting wall on the mixing chamber side directs the liquid radially outwards, which liquid preferably flows axially into the mixing chamber. On the combustion chamber side, the narrowing leads to a loss-free guiding of the flow of gas into the pressure equalizing tube.